Wafers of silicon carbide of 4H and 6H polytypes serve as lattice-matched substrates to grow epitaxial layers of SiC and GaN which are used for fabrication of SiC- and GaN-based semiconductor devices. The performance of such semiconductor devices is strongly affected by crystalline defects in the substrate and epilayer. Among the most unwanted defects in the SiC substrate are micropipes, dislocations and low-angle grain boundaries. It is generally recognized that high defect densities in SiC and GaN single crystals adversely affect the performance of devices made from these single crystals.
With reference to FIG. 1, large SiC single crystals are commonly grown by a sublimation technique called Physical Vapor Transport (PVT). A typical PVT growth system includes a crucible 1, typically made of graphite, loaded with polycrystalline SiC source material 3 and a SiC seed crystal 4, typically, a SiC single crystal. Source 3 is disposed at the bottom of the crucible 1 while seed 4 is affixed to a crucible lid or top 2. Crucible 1 is surrounded by thermal insulation 6. An RF coil 7 couples electromagnetically with crucible 1 heating it to SiC growth temperatures, generally between 1900° C. and 2400° C. Also or alternatively, a resistance heater (not shown) can be utilized for heating the interior of crucible 1 to SiC growth temperatures. RF coil 7 is positioned with respect to crucible 1 in such a fashion that the temperature of source 3 is higher than the temperature of seed 4, with the temperature difference being between several and 200° C. Source 3 and seed 4 temperatures are typically monitored using optical pyrometers via openings 8 made in thermal insulation 6.
At high temperatures, source 3 vaporizes and fills crucible 1 with volatile molecules of Si2C, SiC2 and Si. Driven by the temperature gradient inside crucible 1, the vapors move and condense on seed 4 forming a single crystal 5.
Prior art methods and apparatus for PVT growth are disclosed in the following documents: Y. Tairov and V. Tsvetkov, “Investigation of Growth Processes of Ingots of Silicon Carbide Single Crystals”, J. Crystal Growth, Vol. 43 (1978), pp. 209-212; D. Nakamura et al., “Ultrahigh-Quality Silicon Carbide Single Crystals”, Nature 430, pp. 1009-1012, 2004; D. Nakamura et al., “SiC Single Crystal, Method for Manufacturing SiC Single Crystal, SiC Wafer Having an Epitaxial Film, Method for Manufacturing SiC Wafer Having an Epitaxial Film, and SiC Electronic Device”; I. D. Matukov et al., “Faceted Growth of SiC Bulk Crystals”, Mat. Sci. Forum 457-460 (2004), pp. 63-66; and U.S. Pat. Nos. 5,683,507; 5,611,955; 5,667,587; 5,746,827; 5,968,261; 5,985,024; 6,428,621; 6,508,880; 6,534,026; 6,863,728; 6,670,282; 6,786,969; and 6,890,600.
Dimensional defects in hexagonal 4H and 6H SiC crystals, such as single crystal 5, can be divided into two categories: “Threading” and “Basal Plane”. Threading dislocations are those with the dislocation line parallel to the hexagonal c-axis. Examples of threading defects include Threading Edge Dislocations (TED), Threading Screw Dislocations (TSD) and micropipes. Basal Plane Dislocations (BPD) and basal plane stacking faults are those laying parallel to the basal c-plane.
When a SiC crystal is grown in the c-direction (also known as “on-axis” or “normal” growth), TEDs, TSDs and micropipes present in the seed crystal replicate and propagate into the growing crystal, while basal plane defects do not. When a SiC crystal is grown in the direction perpendicular to the c-axis (also known as “lateral” growth that can be carried out in the a, m or other crystallographic directions perpendicular to the hexagonal c-axis), the situation is opposite: basal plane defects (BPDs and stacking faults) replicate and propagate into the growing crystal, while threading defects do not. Therefore, repetition of lateral and normal growth can lead to reduced defect densities and improved crystal quality.
In one prior art growth technique, lateral and normal crystal growth is carried out separately, each time requiring a new seed, new source, new crucible, etc. However, this technique is too complex and lengthy to be used as an industrial process.
With reference to FIG. 2, a sequence of lateral and normal growth takes place when the growing crystal expands laterally beyond the seed size. A SiC seed 10, like SiC seed 4 in FIG. 1, with its faces cut parallel to the crystallographic c-plane contains threading defects (dislocations, micropipes) shown as dotted lines in FIG. 2, which also depicts three sequential time intervals of crystal growth designated as Step 1, Step 2 and Step 3. In Step 1, the crystal grows normally from seed 10 forming a crystal layer 11. Layer 11 inherits all threading defects from seed 10, but contains no basal plane defects. Simultaneously with the growth of layer 11, volume 12 is formed. Volume 12 grows laterally from layer 11 and, therefore, is free of threading defects. However, it is also free of basal plane defects because layer 11, which serves as a seed for volume 12, does not contain them.
In Step 2, a layer 13 grows in a direction normal from layer 11, inheriting all threading defects. Simultaneously, volumes 14 and 15 are formed. Volume 14 grows in a direction normal from volume 12, while volume 15 grows laterally. Both volumes 14 and 15 contain neither threading nor basal plane defects.
In Step 3, volumes 16, 17 and 18 are formed. As can be seen, volume 16 contains threading defects, while volumes 17 and 18 contain neither threading nor basal plane defects.
As can be seen, the SiC boule portion that expanded laterally beyond the limits of seed 10 includes volumes 12, 14, 15, 17 and 18 that ideally contain neither threading nor basal plane defects.
Two possible modes of crystal diameter expansion, free and guided, are shown schematically in FIGS. 3A and 3B, and 3C, respectively. FIGS. 3A-3C each show an isolated view of the upper portion of a growth crucible (like the growth crucible shown in FIG. 1) with a SiC seed crystal attached to the crucible lid or top. With particular reference to FIGS. 3A and 3B, in free expansion, the single crystal does not touch the crucible walls or any other interior parts of the crucible (except the crucible top) and its shape and morphology are determined only by temperature gradients in the crucible. In the case when the radial temperature gradients are strong, the crystal develops a shape of a lens with multiplicity of edge facets, as shown in FIG. 3A. This type of expansion leads to a high degree of thermo-elastic stress and generation of dislocations, especially near the edge of the crystal boule. When the radial temperature gradients are low, polycrystalline SiC nucleates on the crucible lid around the seed and grows side-by-side with the single crystal, as shown in FIG. 3B, thus making the degree of single crystal expansion unpredictable.
A schematic diagram of so-called guided expansion of SiC boule diameter is shown in FIG. 3C. The distinctive feature of this technique is the cone-shaped growth guide which surrounds the seed and which forces the crystal to attain the shape of the guide's inner cavity in a process known as guided diameter expansion.
With reference to FIGS. 4A-4D, an ideal case of guided diameter expansion shown in FIG. 4A occurs when a SiC crystal, during growth, “glides” along the inner surface of the guide without coming in contact with it. A small gap, usually 1-2 mm wide or less, exists between the crystal and the growth guide.
However, the technique of guided expansion, is not problem free. The most harmful problems are illustrated in FIGS. 4B, 4C and 4D. FIG. 4B depicts erosion of the growth guide caused by the aggressive silicon-rich vapor(s) produced in the crucible during PVT growth. As a result of erosion, the inner surface of the growth guide becomes irregular, and so does the shape of the grown single crystal. The zone of irregular growth near the edge of the grown single crystal is populated with numerous defects.
Another problem is the formation of polycrystalline SiC deposits on the growth guide, as shown in FIG. 4C. These deposits consume nutrients from the vapor phase, thus reducing the size of the growing single crystal.
A merger between the crystal and the growth guide is illustrated in FIG. 4D. In the process of growth, the single crystal comes in contact with the guide, attaches to it and develops crude defects at the edge.
It is believed that in order to eliminate the merger of the single crystal with the growth guide as well as deposition of polycrystalline SiC on the growth guide, the inner surface of the growth guide should be maintained at a temperature higher than that of the growing crystal. This can be achieved by tailoring the geometry of the heater or coil, such as RF coil 7 in FIG. 1, and/or by using a growth crucible of a special (and complex) geometry.
PVT growth is a “closed” process, and the temperature inside the crucible cannot be measured experimentally—it can be assessed only by modeling. At high temperatures, thermal properties of graphite and SiC crystal are known only approximately. This makes the accuracy of thermal modeling low. In addition, thermal conditions in the crucible change during growth.
Erosion of the growth guide is caused by chemical attack of the guide material (usually graphite) by the aggressive silicon-rich vapor(s) produced in the crucible during PVT growth. Typically, crucibles, heat shields and other parts used in SiC sublimation growth are made of high-purity isostatically molded graphite. It is well-known that the chemical resistance of high-purity graphite depends on its structure. Generally, graphite is prepared by mixing graphitic filler (coke) with binder. Graphitic coke is comprised of small-size graphite grains ranging from tens to hundreds of microns in dimensions. The binder is comprised of an oil residue (pitch, tar) or of a high-carbon resin. The prepared mixture undergoes carbonization and graphitization, the latter at temperatures up to 3000° C. The final structure after graphitization is comprised of graphite grains surrounded by the graphitized binder. The graphitized binder is still somewhat amorphous, i.e., it contains disordered chemical bonds. In the process of chemical attack, the graphitized binder erodes (is removed) first. Removal of the graphitized binder liberates microscopic graphite grains, which may become airborne and contaminate the growing SiC crystal. This makes graphite with high binder content more prone to gas/vapor erosion than graphite with low binder content.
In order to protect graphite against gas/vapor erosion, various carbon and refractory coatings have been suggested. Carbon coatings may be comprised of amorphous (glassy, vitreous) carbon or pyrolytic crystalline graphite. These carbon coatings are thin, typically not more than 40-50 microns in thickness, and, in the conditions of SiC sublimation growth, they are quickly eroded by the vapor. The protective refractory coatings may include tantalum carbide or niobium carbide. However, in the conditions of SiC sublimation growth, these refractory coatings are not inert. Rather, they react with the vapor, peel off and lead to crystal contamination.